The present invention relates to “active implantable medical devices,” as defined by the 20 Jun. 1990 Directive 90/385/EEC of the Council of the European Communities. More particularly, the invention concerns implants that continuously monitor a patient's heart rhythm and deliver to the heart, if necessary, electrical pulses of stimulation, resynchronization, and/or defibrillation in the event of rhythm disorder detected by the device.
Illustrative of the devices in question is a device with an antitachycardia therapy mode, to terminate a tachyarrhythmia, which involves the controlled application of defibrillation shocks in the form of high-energy electrical pulses, and/or antitachycardia pacing (ATP) and the like. Such devices are called “implantable cardioverters/defibrillators” or “LCDs”.
The decision to apply a therapy may be made by an algorithm configured to analyze the heart rhythm and detect a tachyarrhythmia episode. The various disorders may be detected and classified according to several discrimination criteria such as the ventricular frequency, the stability of ventricular intervals (RR intervals), the analysis of the atrioventricular association (revealed by the stability of the PR interval), the starting mode of the tachycardia (presence of a sudden acceleration and ventricular or atrial origin cavity) or the morphology of the intracardiac signal.
The rhythm analysis and, hence, the decision to deliver or not to deliver a therapy may be affected by artifacts collected by the lead, however. Indeed, these implants are sensitive to the detection of non-cardiac origin signals due to problems with the lead, to electromagnetic interference, or to the detection of myopotentials, etc. These phenomena are designated hereafter by the generic term of “noise.”
Such phenomena are likely to generate artifacts, the detection of which by the implant may have very serious consequences. Thus, the detection of noise can result in the creation of a ventricular pause due to the inhibition of ventricular pacing, which may be symptomatic for dependent patients, or in the onset of delivering inappropriate defibrillation shocks.
Yet, the application of any defibrillation shock in a conscious patient is extremely painful and distressing, with the energy applied being far beyond the pain threshold. In addition, the application of a defibrillation shock is not without side effects on the heart rate (i.e., risk of development of secondary disorders), on the functional integrity of the myocardium, and in general on the physiological balance of the patient. It is important, therefore, to deliver such shocks appropriately.
Various techniques have been proposed to reduce the impact of noise, including the automatic adjustment of the sensitivity of the sensing amplifiers, or the automatic gain control of these amplifiers. Firstly, to detect ventricular fibrillation (VF), the signal level of which is low, it is necessary to have maximum sensitivity; otherwise there is the risk not to detect events that should have been (“sub-detections”). Secondly, however, the ventricular fibrillation signal amplitude (e.g., QRS complexes indicative of ventricular depolarization) may be at a variable level, intermediate between the noise and the sinus complex signals.
During detection of ventricular fibrillation, the detection of a possible noise thus is unavoidable. In other words,                if the implant is programmed to a too high sensitivity value, that is to say, with a detection threshold too low, then ventricular arrhythmia episodes are properly detected but the increased sensitivity to noise greatly increases the risk of applying inappropriate and undesirable therapy (false positive situation, called oversensing).        Conversely, if the implant is programmed to a too low sensitivity value (i.e., with a detection threshold that is too high), then the actual episodes of ventricular arrhythmias may not be detected (i.e., a sub-detection, or false negative, situation), which for the patient can mean very serious consequences.        
Because the detection of extrinsic noise usually is inevitable, the problem addressed by the present disclosure is to adjust the parameters of ventricular arrhythmia detection so as to discriminate noise from cardiac depolarizations, in order to avoid triggering inappropriate therapies or, conversely, inhibiting therapies that would have been justified.
Various proposals have been made in the prior art to try to find a solution to this problem:
(A) United States Published Application No. 2011/0172727 proposes, in cases of probable arrhythmias, to run an “inappropriate detection suspicion” algorithm, based on the number of short RR intervals detected, the coupling between the cavities (PR interval), the morphology of the ventricular depolarization signals, and the instability of the RR coupling. Upon actually suspect detection, the device increases the persistence, that is to say, the latency for the application of a therapy, or modifies the applied therapy (reduction in the number of shocks or changes in the sequence of ATP stimulation). The disadvantage of this technique is that, in case of noise detection, the consequences of inappropriate therapy are only reduced but this therapy is not prevented. In addition, the discrimination technique disclosed does not work in case of high amplitude sustained noise.(B) U.S. Pat. No. 7,953,488 proposes, in case of detected noise or of ventricular undersensing, to perform a step of confirmation by operating a switch of the detection channel to verify that the noise is not present on the other channel. This technique obviously works in a situation in which noise is present on only one of two paths; otherwise, the risk of application of inappropriate therapy remains.(C) U.S. Pat. No. 7,937,135 provides for adjusting the detection sensitivity according to the amplitude of the signal, on analyzed two ECG channels. In cases of suspected noise, a window is opened in which the morphology and the amplitude of the signal are analyzed. If noise is detected then the measured frequency is corrected (short couplings are disabled) to avoid inappropriate diagnosis. This technique is not suitable in cases of high amplitude noise. It also increases the risk of non-detection of ventricular fibrillation in case of an error on the discrimination between noise and VF.(D) U.S. Pat. No. 7,756,570 relates to increasing of the detection sensitivity in the absence of noise and reducing the sensitivity when noise is detected. After a predetermined period (e.g., one or more stimulated cycles), the sensitivity returns to its minimum value, to check whether the noise still is present or whether an arrhythmia has not occurred in the meantime. The detection threshold is a function of the amplitude of the R wave. The amplitude threshold for the detection of noise is thus an adaptive, variable threshold, updated according to the level of the detected noise. The disadvantage of this technique is the delay that it introduces in the detection of an arrhythmia, due to the application of high sensitivity after a fixed period of time, with a risk of detecting noisy cycles when high sensitivity is restored (ventricular pause situation).(E) United States Published Application No. 2002/0165587 operates in a manner comparable to (D), identifying at each cycle a noise level and a signal level and adjusting the detection threshold based on these respective levels. The same problems pertain, however. United States Published Application No. 2004/0015192 also describes the use of a variable dynamic threshold to improve the conditions for delivery of a defibrillation shock.
Other techniques have been proposed to reduce the impact of electromagnetic noise interferences. For instance, United States Published Application No. 2011/0196247 operates a counting of the signal fluctuations in a time window defined between two cardiac events and considers the presence of noise depending on the result of this counting. United States Published Application No. 2009/0326600 proposes, in case of suspected arrhythmias, an impedance measurement of the electrodes to discriminate between arrhythmia and noise.